The microwave integrated circuit is a miniature form of microwave circuit used in advanced microwave radar and communication systems. Microwave integrated circuit structures represent an alternative to more conventional waveguide or coaxial based microwave circuit approaches that meets the need for miniaturization required by the space and weight limitations of modern adaptive radar and communication equipment. Such equipment uses many active channels to achieve system flexibility and complex data processing capabilities. The microwave integrated circuit concept exploits the small size of solid-state devices and their compatibility with a planar form of transmission line. Microwave integrated circuits are thus able to combine the advantages of conventional integrated circuits, e.g., small size, low weight, improved reliability, and reproducability at potentially low cost, with the bandwidth performance benefits of direct integration into the microwave integrated circuit of devices with low parasitic impedance.
A planar transmission line with acceptable microwave propagation characteristics for connections between circuit elements is a necessary provision for microwave integrated circuits. One type of transmission line commonly used in microwave integrated circuits is the microstrip line. The microstrip line typically includes a strip conductor deposited onto a flat dielectric substrate, the reverse side of which is metalized to provide a ground plane. Another type of transmission line used in microwave integrated circuits is the coplanar waveguide transmission line. The coplanar waveguide line includes a center strip conductor with ground planes located on each side of and in parallel with and in the plane of the center conductor strip. Other forms of microwave transmission lines used in microwave integrated circuits include the slot line, the fin-line, and variations of the microstrip line. The planar microstrip, coplanar waveguide, and other microwave transmission lines, are also ideally suited for the realization of printed circuit topologies such as filters, directional couplers, planar resonators, and other quite complicated circuit topologies.
Planar microwave transmission line circuit topologies, such as microstrip and coplanar waveguide transmission line circuit topologies, are fabricated using either a thin-film or thick-film process. Either process may be used to form metal transmission line circuit topologies on a substrate. Typical substrates can be of an inorganic, (e.g., ceramic), synthetic, semiconductor, or ferrite material.
The thin-film fabrication process may be implemented by what is called a liftoff process, or by a process which utilizes metallization, patterning, and metal etching. In the thin-film fabrication process, vacuum-evaporation evaporation (resistance-heating or electron-beam methods) or sputtering techniques are used to deposit a thin layer of metal on a substrate surface. Metalization of the substrate surface is usually achieved by first depositing a very thin film of chromium or nickel, to provide good adhesion, and then applying a thin film of gold or nickel by a vacuum evaporation technique. For microstrip transmission line circuit topologies, both sides of a substrate surface will be metalized, one metalized side of the substrate forming the ground plane. For coplanar waveguide transmission line circuit topologies, only one side of the substrate will be metalized. The thickness of the deposited metal is normally in the 0.1 to 10 .mu.m range. A photoresist layer is then applied over the metalized surface upon which the transmission line circuit topology is to be formed. The photoresist is typically dipped, sprayed, or spun on to the substrate using a centrifuge. Thin and uniform photoresist layers are obtainable by spinning on the photoresist, but the technique can only be used for small substrate sizes. The photoresist must typically be cured at a high temperature after it is applied to the metalized substrate.
A photographic mask, containing the details of the circuit topology to be fabricated, is then aligned onto the substrate, on top of the photoresist. The photoresist layer is then exposed to ultraviolet rays through the mask. The UV radiation must be parallel, and the photoresist layer thin, to ensure an accurate reproduction of the circuit topology pattern. By developing in a suitable chemical, either the UV exposed part of the photoresist layer (positive photoresist) or the non-exposed part (negative photoresist) is removed, exposing unwanted portions of the metalized surface beneath. The unwanted metal is then removed from the substrate by etching (or by a liftoff process). The structure is exposed to an acid that dissolves the exposed metal, but does not affect the remaining photoresist. In this manner, the transmission line circuit topology is formed on the substrate surface. The remaining photoresist is then removed with a solvent. The remaining metallic layer, now the transmission line circuit topology structure, may be thickened by electrolytic deposition of metal, or by a protective layer (for instance, gold) deposited onto the metal to prevent oxidation. All of the steps in the thin-film fabrication procedure must be carefully separated by rinsing and cleaning operations, sometimes followed by drying in an oven. Drying is particularly critical for synthetic substrates, since some of them tend to absorb water.
The objective of the thin-film fabrication technique is to realize a circuit topology having a pattern as similar as possible to the structure defined by the photographic mask. However, during the etching process, the acid does not remove the metal uniformly, but yields a trapezoidal cross-section instead of the rectangular one desired. This phenomenon of under-etching also takes place in the metal deposition process. Under-etching becomes more significant for thicker metal layers; hence, the thin-film fabrication process is only typically used to fabricate thin transmission line conductors and circuit topologies. As a result, the power handling capability of transmission line circuit topologies fabricated using the thin-film method are limited.
The thick-film fabrication technique is used to fabricate planar transmission line structures, resistors, and capacitors. Typical metals used in the thick-film processing technology may include silver, gold, palladium-gold, platinum-gold, and silver-palladium. The thick-film fabrication technique employs a screen that is covered with a photosensitive emulsion. The screen and emulsion are exposed to radiation in a pattern, and the emulsion developed, to form an image of the required transmission line circuit topology structure on the screen. The metal is applied to the substrate by spreading a paste containing the metal and a binder (typically glass beads or particles) through the screen and onto the substrate. The developed emulsion on the screen prevents the paste from passing through the screen in certain areas. Thus, the desired transmission line circuit topology structure is formed on the substrate in the desired pattern. The film of metal paste is then fired at a high temperature (about 800.degree. C.) to drive off the glass binder.
The thick-film fabrication technique normally results in a line resolution of about 100 .mu.m, which is difficult or impossible to use with coupled transmission line topologies requiring spacings of less than 100 .mu.m, narrow high-impedance transmission lines, or precisely defined circuit elements. In addition, the conductivity of thick-film fabricated conductors tends to be considerably less than the conductivity of similar bulk materials, and this can lead to problems of high transmitted-power loss at microwave frequencies. The thick-film fabrication approach is thus usually not accurate enough for microwave circuits, but is used to realize components like resistors or capacitors. This fact generally leads to a preference for the thin-film fabrication technology for microwave integrated circuit applications.
Deep X-ray lithography involves a substrate which is covered by a thick photoresist, typically several to several hundred microns in thickness, that is exposed through a mask by X-rays. X-ray photons are much more energetic than optical photons, which makes complete exposure of photoresist films feasible and practical. Furthermore, since X-ray photons are short wavelength particles, diffraction effects which typically limit device dimensions to two or three wavelengths of the exposing radiation are absent for mask dimensions above 0.1 .mu.m. If one adds to this the fact that X-ray photons are absorbed by atomic processes, standing wave problems, which typically limit exposure of thick photoresists by optical means, become a non-issue for X-ray exposures. The use of a synchrotron for the X-ray source yields high flux densities, several watts per square centimeter, combined with excellent collimation to produce thick photoresist exposures without any horizontal run-out. Locally exposed patterns should therefore produce vertical photoresist walls if a developing system with very high selectivity between exposed and unexposed photoresist is available. This requirement has been satisfied using polymethylmethacrylate (PMMA) as the X-ray photoresist, and an aqueous developing system. See, H. Guckel, et al., "Deep X-ray and UV Lithographies for Micromechanics," Technical Digest, Solid State Sensor and Actuator Workshop, Hilton Head, S.C., June 4-7, 1990, pp. 118-122.
Deep X-ray lithography may be combined with electroplating to form high aspect ratio structures. To do so requires that the substrate be furnished with a suitable plating base prior to photoresist application. Commonly, this involves a sputtered film of adhesive metal, such as chromium or titanium, which is followed by a thin film of metal which is suitable for electroplating the metal to be plated. In appropriate cases, the use of an initial layer of adhesive metal is not necessary. Exposure through a suitable mask and development are followed by electroplating. This process results, after clean-up, in fully attached metal structures with very high aspect ratios. Such structures were reported by W. Ehrfeld and co-workers at the Institute for Nuclear Physics (KFK) at Karlsruhe in West Germany. Ehrfeld termed the process "LIGA" based on the first letters of the German words for lithography and electroplating. A general review of the LIGA process is given in the article by W. Ehrfeld, et al., "LIGA Process: Sensor Construction Techniques Via X-Ray Lithography," Technical Digest IEEE Solid State Sensor and Actuator Workshop, 1988, pp. 1-4.